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These are not instructions!
Always work safely and under supervision.
Do not attempt the following at all unless you are qualified!
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Vehicle Background:
What we have here is an 88 YJ (see specs). It has a 4.2 and fuel injection from SDS. The guy who sold it to me claimed figures of about 160hp and 180ft/lbs (at the wheels). Who knows, but it was strong enough that I DESTROYED reverse gear without too much effort and in short order (1st is complaining too). I had budgeted replacing the tranny from the moment I purchased my heap.

Rebuild Options/Goals:
I was entertaining many options including rebuilding the 10/5, going to an ax-15, and springing for the NV4500!
My goal with this swap is simple...Get back on the road with a NEW transmission (not rebuilt or yard sourced) that will handle most anything I dish out, and not open a huge can of worms. So I did a mess of research on the NV3550.

Turns out it fit the bill...here's how.

Rebuild Obsticals:
I faced some of the following problems.

Budget

Swap complexity

Actual durability

Future torque handling capacity

Timeframe

Budget:
We all know the prices so I wont bore you with that. Lets just say I cant afford to do this twice.

Ease of swap:
The AX-15 would probably have been just as easy to swap as the 3550 and I'm sure many people are very happy with the 15. I might have gone AX-15 but the ratios/torque capacity is better in the 3550. The 4500 is huge and awesome...but there's a bunch more fab work and drive shaft stuff that needs doing for that swap.

Actual Durability:
Considering my torque requirements (current and future) I have decided that I will never ever go above 300 lb/ft from the engine. Given that I will be using this jeep as a daily driver and mild crawler/tow vehicle, I won't be needing a "trail rated" granny low, or huge torque capacity. The NV3550 is rated 300 lb/ft (conservatively) based on a GVW of 7200 lb.
I think my jeep is, at most, half that weight.

Future Torque Capacity:
If I am correct in assuming my injected 4.2 is producing about 207 ft/lbs at the crank (180x1.15), then it's safe to say I'm in the clear with the current motor. However, assuming the conservative rating of the 3550, if I do swap in a nice new 4.0 and stroke it with the old 4.2 internals, or go with a 3.8 S/C (who knows), I will still be safe.

Timeframe:
I want a "drop-in" tranny upgrade!! Unfortunately those don't come easy. The idea of fabricating for weeks and trouble shooting after that, puts most "granny low" transmissions out of the picture for me. Time is not a huge factor for me, but I don't want to be on my back in a frozen garage, battling with mechanical jigsaw puzzles. The best solution is coming into focus...

2 huge boxes arrived at my door after waiting patiently for my Sept. X-mas. The Tranny in one and the adapters/clutch in the other.

I took a quick inventory of all the stuff..."yup all there....woah....whats THAT?"....A chipped Clutch plate that's what.<Replaced Free of charge by Advance Adapters after some E-mailing and picture taking. (AA is a great company)

Parts...check.
Tools...check.

Started by removing the shifters.

Then the front and rear drive shafts came off.

The rear U-joints were in a bad state. One literally fell apart right away, displaying the roller bearings inside. The other was seized.

Next I lowered the belly pan.
This is about the point when I broke out the ziplocks for labeling and storage of "loose items".

Then I removed all the vac lines, speedo cable, backup light wires, vent tubes, and cobwebs and removed the x-fer case.

Next installment I'll give a step by step on replacing the input gear in the np231 (from 21 to 23 spline).

After removing the planitary retainer on the transmission side of the front half, the planitary and input gear comes out as a unit...then remove the input gear retainer...(note there is no need to remove the large ring gear retainer inside the case)

Then note the position of the input gear thrust washers...

Now you can clearly see the two input gears side by side (new left, old right)...